This recipe comes to us from California chef Judy Rodgers.
The day before you make the gnocchi, check the ricotta for wetness: Place about 2 teaspoons ricotta on a dry paper towel, and wait for about 1 minute. There will be a little wet spot under and around the cheese; but if the cheese has made a wide ring of moisture, it is too wet to use. Place it in a fine sieve or in two layers of cheesecloth, and suspend over a deep bowl to drain for 8 to 24 hours in the refrigerator.
Prepare batter: In a medium bowl, beat ricotta vigorously, then smash a little cheese against the side of a bowl with a rubber spatula. If you can still make out firm curds, press the cheese through a sieve again to break them up. Beat in the eggs, Parmigiano, butter, and salt until light and fluffy.
Form the gnocchi: Add enough flour to a shallow dish to come 1/2 inch up the sides. Using a tablespoon held at an angle, shallow-scoop out 2 to 3 teaspoons batter. Use your fingertip to push the almond-shaped scoop of batter from the spoon into the bed of flour. Gently coat the gnocchi with flour. The gnocchi may have a few wrinkles, dimples or bumps.
Cook the gnocchi: Bring a large wide pot of water to a simmer; add salt. Add one gnocchi to test the batter. It will initially sink but will then swell, roll, and rise to the surface. Cook until the gnocchi is just firm, 3 to 5 minutes from the time it floats, depending on the cheese and the size of the gnocchi. If the gnocchi spreads or starts to decompose, the cheese was probably too wet. You can correct this by beating a teaspoon of egg white into the batter. If the batter was very fluffy but the cooked gnocchi seems heavy, beat in about 1 teaspoon beaten egg. Taste the sample gnocchi, and adjust the seasoning with salt. Continue forming the gnocchi, arranging them on parchment paper dusted with flour. Be sure that they are not touching one another. You can cook the gnocchi immediately or you can refrigerate them, uncovered, for about an hour.
Make the sauce: In a large skillet, melt butter over medium heat. Add sage, and cook until fragrant, about 3 minutes. Add mushrooms, and cook, stirring occasionally, until heated through, about 5 minutes. Season with salt.
Cook the gnocchi in batches, adjusting the heat to maintain the simmering water. Use a slotted spoon to remove gnocchi from simmering water, and pat dry. Add gnocchi to skillet with mushrooms, swirling pan gently to coat with butter. Serve immediately.